Dry plate element and method of forming same



Nov. 26, 1940. F. BRUNKE 2,223,203

DRY PLATE ELEMENT AND METHOD OF FORMING SAME Filed sept. 7, 1939 FEA.

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CARR/.ER ELECTRUDE.

IhvenbcT-'z .Fr-itz B-ur'lke,

H iS Attorhedg;

Patented Nov. 26, 1940 UNITED STATES DRY PLATE ELEMENT AND METHOD F ronMiNG SAME Fritz Bnmke, Berlin-Reinickendorf, Germany, assignor to General Electric Company, a corpora.-

tion of New York Application September 7, 1939, Serial No. 293,812 In Germany September 9, 1938 7Claims.

My invention relates to dry plate elements or cells for rectiflers and the like of the selenium type, in which a semi-conductor layer consisting of or comprising selenium, or consisting of or comprising a material, such as tellurium, functioning similarly to selenium as a semi-conductor, is formed on a carrier electrode or plate without any substantial blocking layer between the semiconductor layer and the carrier electrode, the

blocking layer means required for the rectification being on the free surface of the semi-conductor layer, that is, between the semi-conductor material and the counterelectrode.

It is the general object of the invention to provide, in a rectifier or similar cell of the selenium or like type, blocking layer means, and methods of forming and applying the same, of such character that the load capacity and eiliciency of the cell are substantially increased.

'It has been proposed heretofore to produce dry plate cells of the selenium or like type in which only the so-called natural blocking layer which originates automatically during the forming of the cell, that is, without the introduction of foreign materials, was employed in the process of manufacture of the plates. 0n the other hand it has been proposed heretofore to manufacture dry plate cells in which the semi-conductor layer is provided with an artilicial insulating or blocking layer, for example a layer of varnish. It is not possible, however, by employing either of the foregoing proposed methods to produce a dry plate cell for rectiflers or the like characterized by a high blocking resistance and by small internal resistance in the current iiow direction.

In accordance with the present invention these disadvantages are overcome and substantially improved results are obtained in the use of the dry plate cells by so forming the blocking layer means that a natural blocking layer and a blocking layer artificially formed or applied are caused to act in conjunction in a suitable manner. For this purpose, in accordance with the present invention, a superior blocking layer means is provided 5 by utilizing crystalline sulphur as material for an artificial blocking layer applied on the natural blocking layer.

My invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring to the drawing, Fig. 1 illustrates graphically, in the operation in general of dry plate rectiers or cells. the relation of reverse current, or current in the blocking direction, to

voltage; Fig. 2 illustrates graphically the relation of reverse current to voltage in cells of the selenium type or the like as employed heretofore, and the same relation in cells of the selenium type or the like constructed in accordance with 5 the present invention; and Fig. 3 illustrates a dry plate cell in accordance with the present invention.

As shown in the curve of Fig. l, in the operation of dry plate cells or rectiflers in general, over 10 the lower voltage range the reverse current increases at a low rate and approximately linearly as the voltage increases, and after reaching a critical voltage range or point, Vk, rises very rapidly with further increase of the voltage. 15 Hence it is possible in practice to operate the cell only at voltages below the critical range or point Vi since otherwise damage to the cell would ensue because of too great heating thereof. 'I'herefore, in improving the load capacity and eillciency 20 of such cells, the problem is to ensure that the reverse current shall remain small, as far as possible, at voltages in the operating range, and that the critical voltage range Vx shall be raised to as large values as possible. Further, the means 25 employed to attain these objects should not lead to any considerable rise of the internal resistance of the cell.

Neither of the above-mentioned methods employed heretofore, i. e., the provision of only the 30 natural blocking layer and the provision oi.' only the artificial blocking layer, fullls to more than an insufficient degree the above requirements which are to be met in improving the operation of the rectier cell. In the case of rectiiiers or 35 cells employing only the natural blocking layer on the semi-conductor layer, the sharp rise in the reverse current begins at relatively low volt= age values, of approximately ten volts. If on the other hand only a so-called articial blocking layer, preferably consisting of a highly-insulating varnish or articial resin, is provided on the semiconductor layer, it is then impossible to produce a rectifier or cell capable of being used practically. It is true that the use of such artificial blocking layers causes a reduction of the reverse current at low voltages, in that in practice these blocking layers function as a series resistance, but no substantial displacement of the critical range Vx to higher voltages takes place. Moreover the in- 50 crease of the internal resistance in the positive or flow direction of the current is so great that a rectifier or cell thus constructed has a lower efilciency than one having only a natural blocking layer. 5,5

'sulcient displacement of the critical range 'Vx to higher voltages, and increase of the internal'reslstance of the cell, are still encountereddf, for example, a varnish or artificial resin layer is applied to the natural blocking layer. It has been found that the natural blocking layer has a crystalline, therefore a. rough, structure. By employing a material which removes this roughness of structure, as is the case for example with varnishes and articial resins, no substantial improvement in cell operation'can be obtained.

A blocking layer means which entails none of the above-mentioned disadvantages is obtained in accordance with the present invention by employing crystalline sulphur as material for the artificial blocking layer. When applied in proper manner, then, particularly on the already existing corners and points of the natural blocking layer, the crystalline sulphur becomes attached and crystallizes still further on the latter layer. The roughness of the natural blocking layer is thereby very markedly increased. I have found that vin the operation of rectiers or cells having blocking layer means thus constructed, the desired displacement of the critical range Vk to higher voltages is attained,

By employing the crystalline sulphur layer in conjunction with the natural blocking layer, rectiers or cells of the selenium type or the like are produced wherein not only is the reverse current reduced as compared with such devices employing only the natural blocking layer, but at the same time a displacement of the critical range Vx to higher values by 50 per cent takes place. In the usual selenium type rectifiers or cells the load limit is substantially 18 volts in the blocking direction. The conditions illustrated in curve A of Fig. 2 are typical. At 18 volts, which in curve A is the critical voltage point Ver, a very steep rise in the reverse current curve occurs. For example, the reverse current at 20 volts in the case of a cell having a diameter of 45 mm. amounts to approximately 8O milliamperes. However, as shown in curve B of Fig. 2, in the cells produced in accordance with the present invention, the reverse current at 20 volts amounts only to 10 milliamperes, a value of 80 milliamperes being reached only at voltages approximating 30. Therefore, the rectiiiers or cells constructed in accordance Wi-th the present invention may be loaded higher by 50 per cent than the prior usual rectifiers or cells, or to the range designated,v for example, by Vkz.` The resistance in the positive or ow direction is not thereby increased. On the contrary in cells constructed in accordance with the invention a substantial reduction of the internal resistance has been observed.

Especially favorable results are obtained in practicing the invention when the sulphur layer is vaporized on to the natural blocking layer. The thickness of the sulphur layer is preferably of the order of l-5 cm. However, it is to be noted thatV a crystalline sulphur layer which is properly formed to enable the cell in which it is employed to operate at relatively high Avalues of vol-tage, loses its blocking properties to a very considerable extent if it is subsequently caused to fuse so that a smooth coating results. It is advantageous, therefore, to vaporize the sulphur on to the natural blocking layer in a few seconds, as thereby an unsuitable heating of the semiconductor surface is prevented and a highdegree of uniformity of the crystalline sulphur layer is attained. The requirements of low heating and rapid application, by vaporizlng, Aof the sulphur layer are best met by performing the vaporizing process at reduced pressure.

In Fig. 3 the dry plate rectifier element or cell of the selenium or like type constructed in accordance with the invention comprises a carrier electrode I, of for example 45 mm. diameter and 1 mm. thickness, composed of any suitable material as nickel-plated iron, upon which is fused or otherwise suitably deposited a layer 2, approximately 0.1 mm. in thickness, of selenium or comprising selenium. In a known manner, the selenium coating is next subjected for five hours, under pressure at a temperature of the order of 120 degrees C., to a forming process.- For the formation of the natural blocking layer 3 a two hour heating of the selenium layer in air at approximately 210 degrees C. is required. Next, the crystalline sulphur layer I of approximately -5 cm. thickness is deposited on the natural blocking layer 3 by exposing the element for approximately four seconds to sulphur vapor in a heated' vessel in which a pressure of approximately 1 mm. of mercury is maintained. Finally the sulphur-coated free surface of 'the element is sprayed with Woods metal or a similarly fusing alloy to form the counterelectrode 5. The counterelectrode 5 and the carrier electrode i may be provided with suitable current leads 6 and 1.

It will be understood that the crystalline sulphur layer 4 may be produced in other ways than as above described. Thus the sulphur layer may be formed on the selenium surface by cataphoresis. Further, instead of employing selenium or material comprising selenium as the semiconductor layer 2, known substitute materials therefor such as tellurium or material comprising 'tellurium may be employed.

My inventionhas been described herein in a certain embodiment for purposes of illustration. It is to be understood, however, that the invention is susceptible of various changes and modifications and that by the appended claims I intend to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A dry plate element including a carrier electrode composed of material comprising selenium, a natural blocking layer on said semi-conductor layer, an artiilcial blocking layer composed of sulphur on said natural blocking layer, and a counterelectrode on said artificial blocking layer.

2. A dry plate element including a carrier electrode, a semi-conductor layer on said electrode composed of material of the group of materials comprising selenium and tellurium, a natural blocking layer on said semi-conductor layer, an artificial blocking layer composed of sulphur on said natural blocking layer, and a counterelectrode on said articial blocking layer.

3. The method of producing a dry plate element having a semi-conductor layer composed of a material comprising selenium, and a natural blocking layer on said semi-conductor layer, which includes depositing on said natural blocking layer an artificial blocking layer composedy of crystalline sulphur.

4. The method of producing a dry plate element having a semi-conductor layer composed of a material comprising selenium, and a natural` blocking layer on said semi-conductor layer,

which includes exposing the surface o1' said semig5 conductor layer having said natural blocking layer thereon to sulphur vapor to form on.said natural blocking layer an artificial blocking layer of crystalline sulphur.

5. The method of producing a dry plate element having a semi-conductor layer composed of a material comprising selenium, and a. natural blocking layer on said semi-conductor layer, which includes exposing the surface of said semiconductor layer' having said natural blocking layer thereon to sulphur vapor at a pressure substantially less than atmosphere to form on said natural blocking layer an artificial blocking layer of crystalline sulphur.

6. The method of producing a dry plate element having a semi-conductor layer composed oi' a material comprising selenium, and a natural blocking layer on said semi-conductor layer, which includes exposing the surface of said semiconductor layer having said natural blocking layer thereonV to sulphur Vapor for a period of the order of four seconds to form on said natural blocking layer an artificial blocking layer of crystalline sulphur.

7. The method of producing a dry plate element having a semi-conductor layer composed of material of the group of materials comprising selenium and tellurium, and a natural blocking layer on said semi-conductor layer, which includes depositing on said natural blocking layer an articial blocking layer composed of crystalline sulphur.

FRITZ BRUNKE. 

